1. Field of the Invention
The present invention relates to the minute expansion/reduction processing of an image.
2. Description of the Related Art
For image printing apparatuses (printers), there are various printing methods. For example, in the case of an electrophotographic method for fixing toner to a recording medium (e.g., paper) by heat and a pressure, the paper subjected to the heat and pressure undesirably extends or shrinks slightly. The extension of the paper also causes a proportional extension of a printed image. Such an extension of the paper is most remarkably found in the first fixing. Thus, a paper subjected to a double-side printing causes a slight difference in size between the image printed on the front side by the first fixing and the image printed on the back side by the second fixing. Such a slight size difference must be corrected in the field where the printing accuracy is important.
Various means have been known to correct such a difference in the image size caused by the extension of the paper.
For example, in the case of the above-described electrophotographic method, a polygon mirror is rotated to scan laser light. Thus, the extension in the main scanning direction of drawn laser can be corrected by adjusting the oscillation frequency of the pixel clock. The extension in the vertical scanning direction (sub scanning direction) on the other hand can be corrected by changing the rotating speed of the polygon mirror to thereby change the scan density.
In this case, in order to draw the respective pixels on a photoconductive drum correctly and uniformly, the rotation of the polygon mirror must be accurate and stable. When an approach is used to cope with the extension in the sub scanning direction during the double-side printing by changing the rotating speed of the polygon mirror after the printing of a front side of a paper, a fixed time interval is required until the rotating speed is stable. Thus, a declined printing performance is caused by the correction of the extension in the sub scanning direction by the adjustment of the rotating speed of the polygon mirror. Therefore, the correction of the extension in the sub scanning direction by the change of the rotating speed of the polygon mirror is not desired in such an image printing apparatus that is required to continuously maintain a high-speed printing.
In addition to the method for adjusting the drawing density by the printing mechanism-side as described above, there is another method to adjust the image data itself depending on the extension of the paper. Specifically, this is a method to thin out pixels to reduce the image size or to interpolate (or insert) pixels to expand the image size. For example, in the case of image data having 1000×1000 pixels in the main scanning direction and 1000×1000 pixels in the sub scanning direction respectively, the pixels are thinned out to 1000×990 or pixels are interpolated (or inserted) to convert the data to image data of 1000×1010 pixels. As described above, by generating new image data subjected to expansion/reduction so as to compensate the extension of the image undesirably caused during the printing of the front side, images having the same size in appearance can be printed on both sides of the paper.
The method as described above to correct the image data itself depending on the extension of the paper is desirable in the point that the printing mechanism is prevented from having a deteriorated performance. In the case where multivalued image data is subjected to a correction processing for performing an interpolation processing such as a linear interpolation to expand or reduce the image to output the image through a multiple tone apparatus (e.g., display), it is difficult to visually confirm the deterioration of the image due to the correct.
However, in the case where the corrected image data is outputted as a printed matter on a paper, the deterioration of the image can be undesirably visually recognized because the image data is converted from multivalued image data to binary image data and pixel values subjected to the interpolation or thinning are also binary.
The dots occur periodically by performing dither processing etc. to multivalued image data. The texture is generated by these periodic dots in binary image data. The texture of binary image data is more influenced by the interpolation or thinning of a pixel column than in the case of multivalued image data. Thus, when the same coordinates are subjected to interpolation or thinning in a longitudinal or lateral direction, this is easily recognized as a change in the concentration. Furthermore, when the coordinates of a pixel column to be subjected to interpolation or thinning are identical with the coordinates of a fine line of the texture, a phenomenon is undesirably caused where the pixel width is reduced by one pixel and the fine line changes into a thick line or disappears. Furthermore, when a change is caused in the size of the textures such as halftone dots changes arranged on a single line, this is undesirably recognized as a change in the concentration. To prevent this, in a processing for correcting binary image data, a method as disclosed in Japanese Patent Laid-Open No. S61-206065(1986) for example has been used where a position at which a pixel is inserted or a thinning position is varied randomly to thereby suppress a local change in the concentration.
However, when the method disclosed in Japanese Patent Laid-Open No. S61-206065(1986) is used to subject the entire face of an image to an interpolation or thinning processing, this has caused a disadvantage of an increased length of a boundary between a region including pixels (printing region) and a region not including pixels (a region of a paper background color) or a broken texture in the image. FIG. 13 illustrates an example where the pixels in the entire face of the image are randomly interpolated. Parts shown by circles show inserted pixels. The reference numeral 1301 denotes a status where inserted pixels cause a difference in the boundary among region including pixels and regions not including pixels. The reference numeral 1302 denotes a status where the insertion of pixels causes a significantly-broken shape of a texture that should be represented by a square composed of 4 pixels in the longitudinal direction and 4 pixels in the lateral direction.
The disadvantage of the increased boundary length means, in an image printing apparatus based on the electrophotographic method, an increased boundary length between a toner-attached region and a no-toner-attached region. This consequently causes a declined stability of the toner attachment, thereby causing disadvantageous scattered toner. The broken shape of the texture also causes an image lacking in smoothness.